This invention relates to assemblies of semiconductor devices and more particularly to assemblies of disc-mounted semiconductor devices with provisions for the distribution of compressive forces on the semiconductor pole faces.
Disc mount semiconductor devices are those in which a semiconductor device is encapsulated in a package having opposing circular faces to which pressure contact is made, without bonding, for electrical continuity to regions of the enclosed device as well as for heat transfer. Such packaged devices are well known and widely used and may include either diodes, transistors, or thyristors, in accordance with known practice. The term "hockey puck" is often used in referring to devices encapsulated in this manner because of similarities of appearance. Such devices are also sometimes referred to as compression bonded encapsulated (CBE) devices.
The nature of disc mount devices is such that it is necessary to apply a predetermined force, typically in a range from about 1000 to about 2000 pounds, that is accurately positioned along the central axis of the device. The compressive force is necessary to ensure good conduction of current and heat through the various interfaces within the semiconductor and at the semiconductor pole faces. Uneven forces may cause hot spots or mechanical damage by localized overstressing of the semiconductor parts. The hot spot or overstress will cause premature failure of the semiconductor.
For many years, heat sink vendors have been offering clamps specifically designed for compressing hockey puck semiconductors. Many of the clamps provide a gimbal or swivel to center the compression force. Others provide a deformation in the clamp spring to provide centering of the force. None of the systems offered for sale by these vendors make provisions of uniform distribution of the clamping force over the pole face of the hockey puck semiconductor.
In use, one or more of the semiconductor devices, frequently several, are stacked with intermediate sheet metal elements that serve as heat sinks and electrical conductors. Prior art disc-type semiconductor mounting arrangements employ a clamp which places a compressive force along the central axis of the stack of semiconductors and sheet metal elements. If the sheet metal element which is in contact with one of the semiconductor pole faces is very thick, it will tend to distribute the compressive force from the swivel or gimbal or deformed spring, but the forces will still be higher in the center than near the periphery of the semiconductor pole face. If the heat sink or bus bar is thin, the forces will be highly concentrated near the center.
The present invention comprises a stack of components including one or more disc-mounted semiconductors and one or more force distributing spacers, arranged in line along a common axis, and means for applying compressive force along the axis, such as a clamp. Each force distributing spacer is counterbored on the side which is closest to the semiconductors to cause an outward distribution of the applied force. The opposite side of the spacer may have a swivel, ball, gimbal, convex surface, or other means to ensure that the applied force is at the center of the spacer. The thickness of the spacer need only be adequate to transfer the force without deformation which would allow contact at the center within the counterbore.
This arrangement results in a force distribution on the semiconductor side of the spacer in the form of a narrow band ring around the perimeter of the counterbore. The insertion of a metal heat sink or bus bar between the spacer and semiconductor increases the width of the force band at the semiconductor pole surface, thereby effectively averaging the force in the pole face area.